Monoprice.com compatibility

When I use auto-fill, either on my iPhone or my Mac, on monoprice.com, I always get told that I've used an incorrect login or password. But when I use the "copy password" feature from 1Password and do it the old-fashioned way, it works. I've verified that I've got the correct username (my email address) in 1Password, so I assume this is some kind of incompatibility with how Monoprice does user login.

1Password Version: 8.10.58
Extension Version: 8.10.56.28
OS Version: macOS 15.2
Browser: Firefox 134.0.2